FIG. 1 shows a front perspective view of an inline spectrophotometer (ILS) 10 for making color measurements of substrates 25. For example, the ILS 10 may be an X-Rite® ILS15 radially-arrayed spectrophotometer available from X-rite Inc. (Grand Rapids, Mich.).
The ILS 10 generally includes a housing 11 enclosing a illuminator 12 and a ring 13 of photosensors 14. The ILS 10 may be used to take color measurements, for example, of inks 26 and the substrate 25. A processor (not shown), such as an integrated circuit and/or software, may be provided for analyzing measured color data.
The substrate 25 may be one or more of: 8½×11 inch letter paper, A4 letter paper, or 8½×14 inch legal paper. However, it will be appreciated that other sizes and substrate media types may similarly be used, such as, bond paper, parchment, cloth, cardboard, plastic, transparencies, film, foil, or other print media substrates. The substrate 25 may also be a reference calibration tile.
The substrate 25 may be generally located in an XY target plane spaced below the ILS 10, and may be moved, for example, with a conveying device (not shown), with respect to the ILS 10 in a process direction. An area 27 of the substrate 25 may be illuminated by light 16 produced by the illuminator 12. In one implementation, the illuminator 12 may be a fluorescent lamp, producing broad spectrum of “white” light across the visible spectrum (i.e., from about 400 to 700 nm (in wavelength)). Other illuminators may similarly be used, such as light emitting diodes (LEDs).
Light reflected from the inks 26 or the substrate 25 may be generally directed in various directions, with a portion 16′ thereof (shown) directed toward the ring 13 of photosensors 14 for measurements.
FIG. 2 shows a bottom plan view of the ILS 10 shown in FIG. 1. In the implementation shown, the ring 13 includes sixteen (16) photosensors 14a-14p, such as photodiodes, spaced equally about in a circular manner. In one implementation, the ring 13 many have an inner nominal radius of approximately 1 cm, although it will appreciated that other configurations and/or sizes are possible. The photosensors 14a-14p are positioned radially around the spectrophotometer in the same plane, and may be selectively filtered or otherwise configured to detect light of specific wavelength.
The output of the ILS 10 may be normalized (i.e., calibrated) as is generally known in the art when making color measurements. These measurements are typically fed back to the printer for controlling various aspects of color output.
However, if the plane of the sensor is titled with respect to the surface plane of a measured object, the sensor ILS 10 signal response will be modulated by the cyclically varying distance of the sensors from the measured object surface as a function of the heading angle of the tilt with respect to the heading angle of the sensors.
FIG. 3 shows a plot of measured reflectance values of a white piece of paper using the X-Rite® ILS15. Due to the tilt, signals from photosensors that are further away from the media, may become slightly attenuated, while signals from sensor that are closer to the media may be slightly increased.
This may result in errors in color measurement, and ultimately in color reproduction errors when the sensor is used in a system for color correction feedback.